What is the evidence for, causes of, and
consequences of changes in the Earth's climate since the pre-industrial
era?

Has the Earth's climate changed
since the pre-industrial era at the regional and/or global scale?
If so, what part, if any, of the observed changes can be attributed
to human influence and what part, if any, can be attributed to
natural phenomena? What is the basis for that attribution?

What is known about the environmental,
social, and economic consequences of climate changes since the
pre-industrial era with an emphasis on the last 50 years?

2.1

This answer focuses on classical measures of climate (e.g., temperature,
precipitation, sea level, plus extreme events including floods, droughts,
and storms), on other components of the Earth's climate system (e.g.,
greenhouse gases and aerosols, ecological systems), and on human health
and socio-economic sectors. Climate change as defined in IPCC
refers to statistically significant variations that persist for an extended
period, typically decades or longer. It includes shifts in the frequency
and magnitude of sporadic weather events as well as the slow continuous
rise in global mean surface temperature. Thus the discussion here includes
climate-weather variations on all temporal and spatial scales, ranging
from brief-lived severe storms to seasonal El Niño events, decadal
droughts, and century shifts in temperature and ice cover. Although short-term
climate variations are considered predominantly natural at present, their
impacts are discussed in this question because they represent a class
of changes that may become more prevalent in a future climate perturbed
by human activities (see Question 4).
Attribution is used here as the process of establishing the most likely
causes for the detected change with some defined level of confidence.
The discussion includes both climate change that is attributable to human
influence and climate change that may at present be natural but might
in the future be modified through human influence (see Box
3-1).

2.2

The Earth's climate system has demonstrably
changed on both global and regional scales since the pre-industrial
era, with some of these changes attributable to human activities.

2.3

Emissions of greenhouse gases and aerosols due to
human activities continue to alter the atmosphere in ways that are expected
to affect the climate (see Table 2-1).

2.4

Concentrations of atmospheric greenhouse
gases and their radiative forcings have generally increased over the 20th
century as a result of human activities. Almost all greenhouse
gases reached their highest recorded levels in the 1990s and continue
to increase (see Figure 2-1).
Atmospheric carbon dioxide (CO2) and methane (CH4)
have varied substantially during glacial-interglacial cycles over the
past 420,000 years, but even the largest of these earlier values are much
less than their current atmospheric concentrations. In terms of radiative
forcing by greenhouse gases emitted through human activity, CO2
and CH4 are the first and second most important, respectively.
From the years 1750 to 2000, the concentration of CO2 increased
by 31±4%, and that of CH4 rose by 151±25% (see
Box 2-1and Figure
2-1). These rates of increase are unprecedented. Fossil-fuel burning
released on average 5.4 Gt C yr-1 during the 1980s, increasing
to 6.3 Gt C yr-1 during the 1990s. About three-quarters of
the increase in atmospheric CO2 during the 1990s was caused
by fossil-fuel burning, with land-use change including deforestation responsible
for the rest. Over the 19th and much of the 20th century the terrestrial
biosphere has been a net source of atmospheric CO2, but before
the end of the 20th century it had become a net sink. The increase in
CH4 can be identified with emissions from energy use, livestock,
rice agriculture, and landfills. Increases in the concentrations of other
greenhouse gases -- particularly tropospheric ozone (O3),
the third most important -- are directly attributable to fossil-fuel
combustion as well as other industrial and agricultural emissions.

Where appropriate, the authors of the Third Assessment
Report assigned confidence levels that representtheir collective judgment
in the validity of a conclusion based on observational evidence, modeling
results, and theory that they have examined. The following words have
been used throughout the textof the Synthesis Report to the TAR relating
to WGI findings: virtually certain (greater than 99% chance
that a result is true); very likely (90-99% chance);
likely (66-90% chance); medium likelihood(33-66%
chance); unlikely (10-33% chance); very unlikely
(1-10% chance); and exceptionally unlikely (less than
1% chance). An explicit uncertainty range (....) is a likely
range. Estimates ofconfidence relating to WGII
findings are: very high (95% or greater), high (67-95%),
medium (33-67%), low (5-33%), and very
low (5% or less). No confidence levels were assigned in WGIII.